Optical delay lines and buffers are key components for optical networks and information processing systems. Delay lines based on conventional optical waveguides can be very long. The length can be greatly reduced if the group velocity of light is significantly reduced. “Slow light” can be achieved in engineered structures which bounce light back and forth as it propagates. Such structures can include grating structures as described in more details in references [1], [2], photonic crystal waveguides as described in reference [3], and coupled-resonator optical waveguide (CROW) as described in reference [4]. A major problem in designing delay lines based on such waveguides is the higher-order dispersion, which can cause a distortion of signal. Although the second-order dispersion of grating structures and CROW is zero at the band center, the group velocity can approach zero at frequencies close to the band edges (see references [1], [2], [4].
Two mechanisms have been proposed and used in the past for optical waveguiding. The most widely used is waveguiding by total internal reflection. Another mechanism is the Bragg waveguiding, in which waveguiding is achieved through Bragg reflection from a periodic structure, has also been demonstrated.